The study of open circuit voltage in Ag/Bi0.9La0.1FeO3/La0.7Sr0.3MnO3

heterojunction structure

R.L. Gao a,n, H.W. Yang a, Y.S Chen a, J.R. Sun a, Y.G. Zhao b, B.G. Shen a

a Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Science, Beijing 100190, Chinab Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o

Article history:Received 19 July 2013Received in revised form12 September 2013Accepted 19 September 2013Available online 26 September 2013

Keywords:Open circuit voltageBiFeO3

HeterojunctionLight illumination

a b s t r a c t

Bi0.9La0.1FeO3 thin films have been grown on La0.7Sr0.3MnO3/SrTiO3 substrates by using pulsed laserdeposition with Ag as the top electrode. The intrinsic open circuit voltage (Voc) (without light illumination)as large as 5.8 V was observed in this hetero-junction structure. Influences of electric pulse treatment, lightillumination and measurement duration time on the Voc were studied, significant Voc improvements wereobserved after shortening the measuring duration time and/or consistent negative pulses, yet Voc droppedobviously with consistent positive pulses and/or with light intensity increasing. Two stable states with highand low Voc can be switched with optical switching, this may be useful in practical application.Accumulation of oxygen vacancies with positive charges near the interface is proposed to explain theseresults.

& 2013 Published by Elsevier B.V.

1. Introduction

Multiferroic materials, which simultaneously display severaltypes of order, such as ferroelectricity and magnetism, haverecently become the focus of research because of the intriguingscience behind their magnetoelectric coupling phenomenon andtheir exciting application potential in multiple controlled devices[1–3]. Among them, single-phase multiferroic BiFeO3 (BFO) is themost extensively investigated due to its high ordering tempera-tures, namely, a ferroelectric Curie temperature (TC�1100 K) andan antiferromagnetic Néel temperature (TN�640 K).[3,4] Sucha specific feature—that both TC and TN in BFO are above roomtemperature—is of great significance from a technological point ofview. Moreover, high remnant polarization (Pr�100 mC/cm2), rela-tively smaller band gap near 2.8 eV compared to other ferroelec-trics, is considered as a perspective candidate for technologicallydemanding applications including nonvolatile memories, solar cellsand sensor [1–5]. Recently, photovoltaic effects have been observedboth in BFO crystal and thin films under illumination of visible light[6–11], high external quantum efficiencies of up to 10% whenilluminated with the appropriate wavelength was also reported[10,11]. The photovoltaic properties are completely different fromthose of conventional p–n and Schottky junctions originating fromthe built in field induced by space charge in depletion layers.

Remarkable photovoltaic effect can be observed under white lightillumination or other lasers for the wavelengths smaller than theoptical energy gap of BFO. The light-induced open circuit voltage(Voc) is not limited in the band gap of BFO, and it can exceed theband gap several multiples, which can be attributed to the smalloptical band gap and narrow domain walls. Yang et al. [10] reportedabout 20 V of Voc in BFO film, which is considerably larger than theband gap of BFO.

However, in previous works, Voc was observed only under lightillumination, and the value of Voc indeed increased with lightintensity and Voc was negligible in dark. Herein, we report anopposite result with oxygen deficient Bi0.9La0.1FeO3 films, severalvoltages of open circuit voltage (Voc) without illumination isobserved and Voc decreases with light intensity increasing. Sig-nificant Voc improvements are observed after prolonging themeasuring duration time or consistent negative pulses; however,Voc decreases with consistent positive pulses or light intensityincreasing. Accumulation of oxygen vacancies with positive chargesnear the interface is proposed to explain this phenomenon.

2. The experimental process

Bi0.9La0.1FeO3 (BLFO) thin films with 500 nm thickness weredeposited epitaxially on (0 0 1) SrTiO3 (STO) substrates using thepulsed laser deposition method (PLD). The deposition process wasdepicted elsewhere [12]. For the study of electrical properties, theconductive metallic oxide La0.7Sr0.3MnO3 (LSMO) layer serves asthe bottom electrode. LSMO was chosen as bottom electrode

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0921-4526/$ - see front matter & 2013 Published by Elsevier B.V.http://dx.doi.org/10.1016/j.physb.2013.09.033

n Corresponding author. Tel.: þ86 10 82648085; fax: þ86 10 82649921.E-mail addresses: gaorongli@iphy.ac.cn (R.L. Gao),

yschen@aphy.iphy.ac.cn (Y. Chen), jrsun@iphy.ac.cn (J.R. Sun).

Physica B 432 (2014) 111–115

because its lattice parameter (LSMO�0.388 nm) is very close toSTO (�0.391 nm), BFO (�0.396 nm), and therefore, with smalllattice parameter mismatch and stress, one expects the epitaxialgrowth. First, a sintered stoichiometric LSMO target was employedfor deposition of LSMO thin films on STO substrates under adeposition temperature of 700 1C, and an oxygen pressure �50 Pa.Then, the BLFO thin films were grown at 650 1C under a lowoxygen pressure of 15 Pa. The other laser parameters during deposi-tions were (i) laser source: KrF excimer laser with λ¼248 nm,(ii) repetition frequency: 5 Hz, (iii) energy density was about 1.5J/cm2. Typical film growth rate was around 9 nm/min. After deposi-tion, BLFO films were slowly cooled to room temperature (2 1C/min)in the oxygen atmosphere of 100 Pa. For measuring the electricalproperties of the films, a 200 nm thick Ag layer patterned with0.2 mm diameter was deposited on BLFO as the top electrodes byPLD through a shadow mask. The thickness was controlled bydeposition time after standardization. The current–voltage measure-ments in the dark and under illuminationwere performed by using aKeithley 2611 source meter. In the measurement process, Ag elec-trode was connected to the positive pole and LSMO as bottomelectrode was connected to the negative pole of the voltage source.Green laser with wavelength of 532 nmwas applied to illuminate onthe Ag top electrode, the laser spot is about 5 mm2, larger than thearea of Ag electrode. In the process of applying electric pulses, Ag topelectric was also connected to the positive pole of the pulse signalgenerator and LSMO was connected to the negative side. The pulseamplitude was 40 V with the period of 12 μs.

3. Results and discussion

Fig. 1(a) shows a typical I–V curve in dark, the measuresequence is 0-6 V-0 V-�6 V-0 V. Fig. 1(b) is the enlargement

of the rectangle area of Fig.1(a), the arrows indicate the sweepdirection of the bias. It can be seen from Fig. 1(b) that the I–V curveshows a large open circuit voltage (Voc) of 2.82 V rather thana negligible value in the dark reported by other authors. This bigVoc plays the role of cell and it may result from oxygen vacanciesaccumulation near Ag/BLFO interface. Therefore, Voc shoulddecrease if positive electric pulse applying to the top Ag electrodebecause oxygen vacancies can be repelled away from Ag/BLFOinterface and injected into BLFO film. In order to study howdifferent voltages affect Vov, I–V curves with different positivepulse voltages were plotted in Fig. 1(c). To obtain the open circuitvoltage directly, the I–V curve was plotted in semilog coordinate.It is found that Voc decreases with the number of the pulse voltageincreasing, its value decreases from 4.2 V in the original state to3.9 V after 50 positive pulses, and 3.35 V after 2000 positivepulses, as shown in Fig. 1(c). However, consistent negative electricpulses play the opposite role, the open circuit voltage increasesfrom 3.03 V in the original state to 4.96 V after 50 negative pulsesvoltage, and 5.76 V after 2000 negative pulses voltage, as shown inFig. 1(d). It can be found from Fig.1 that the intrinsic open circuitvoltage Voc is always positive, no matter the voltage applied on theAg electrode is positive or positive. However, the value of Voc

exhibits strong dependence of electric pulse directions, Voc

decrease/increase with positive/negative pulse voltages, it maybe that as pulse voltages can affect the accumulation or distribu-tion of defects, such as oxygen vacancies with positive charges,and it therefore affects the Voc.

In order to analyze how the distribution or the movement ofoxygen vacancies affect Voc, the value of the sweeping durationtime from þ6 V to 0 V in the I–V curve was controlled from 0.5 s to12 s, Voc as a function of sweeping duration time was shownin Fig. 2(a) and (b). It can be concluded from Fig. 2(b) thatVoc decreases with increasing sweeping duration time. Its value

Fig. 1. (a) I–V curves of Ag/BLFO/LSMO hetero-junction in the dark, (b) enlargement of the rectangular area in (a), (c) I–V curves with different consistent positive pulse, and(d) I–V curves with different consistent negative pulse. The arrows indicate the sweep direction of the bias.

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decreases from 5.5 V to 2.1 V when the sweeping duration timevaries from 0.5 s to 12 s. This result indicates that Voc depended onthe accumulation of positive oxygen vacancies, with the sweepingduration time increasing, more and more positive charges willmove away from Ag top electrode and ejects into LSMO bottomelectrode in the discharge process. As evidenced in Fig. 2(b), afterthe first 2 s it looks like the beginning of exponential decay to zero.Because Voc is obtained from the I–V sweeping curves, thereforethe positive sweeping voltage also has distinct effect on Voc, asshown in Fig. 2(c) and (d). It can be seen from Fig. 2(c) and (d) thatVoc increases from 0.77 V to 5 V as the sweeping voltage increasingfrom 1 V to 6 V; however, it decreases to 4.29 V when thesweeping voltage increases to 7 V. This may be that larger positivevoltage can repel more positive oxygen vacancies away from Ag/BLFO interface and inject into the negative pole side, results thedecrease of Voc.

Different open circuit voltages induced by visible light or byultraviolet light (UV-light) have been observed in BFO singlecrystal and thin films observed [13–18]. All of these results abovereported indicated that the value of Voc increased with lightintensity. It can be seen from Fig. 3(a) that light illuminationindeed obviously impacts on Voc; however, the most obviousdifferences from other reports is that, after illumination, Voc

reduces rather than increasing, its value decreases from nearly 3 Vto nearly zero. It is worth pointing out that two stable states withhigh and low Voc can be switched with the light on and off, this maybe useful in practical application. Fig. 3(b) shows the effect of lightintensity on the Voc, it can be seen form Fig. 3(b) that Voc decreaseswith the light intensity increasing, and the variation of Voc versuslight intensity follows an exponentially attenuation law as shown inFig. 3(c). Meanwhile, Voc can reduce to near zero but never benegative in the downward polarization state, in contrast, in the

upward polarization state, Voc can reduce from positive value tonegative value under illumination, this downward-upward statesdependence of positive-negative Voc was observed in most cases[13–17].

The variation of Voc can be understood based on the Ag/BLFO/LSMO energy band diagrams as shown in Fig. 4. To construct theband diagram, the work functions of Ag, LSMO (4.26 eV and4.96 eV), and the electron affinity/band gap of BFO (3.3 eV/2.8 eV), [18,19] a band diagram can be depicted for the Ag/BLFO/LSMO device, here BLFO was treated as an n-type semiconductor.As shown in Fig. 4, the barrier height of the BLFO/LSMO junction ishigher than that of the Ag/BLFO junction, which explains why thecurrent level is low/high under positive/negative bias showed inFig. 1(a). In our previous work [12], we found that all of our as-grown Ag/BLFO/LSMO thin-film capacitors exhibited a one-sidediode effect. Both with the negative poling (i.e., upward polariza-tion) and positive poling (i.e., downward polarization), the I–Vcurve clearly indicated a diode effect with negative forward bias,called a reverse diode. Similar one-side diode effects have beenreported with numerous ferroelectric thin-film capacitors [20–24].Lee et al. [24] attributed this lack of switchability to the formationof an interfacial defective layer at the Pt/BiFeO3 top interface, theyshowed that all of their as-grown BiFeO3 thin films on SrRuO3

bottom electrodes were self-poled downward so that the negativepolarization charge was built near the top BiFeO3 surface [5,25,26].According to PFM images, the as-grown BLFO thin films on LSMObottom electrodes show nearly the same phase as that polarizedby positive voltages, indicates self-polarized and its direction isdownward, as shown in Fig. 5. Therefore, during the high-temperature film deposition, the downward self-polarization willcause the positively charged oxygen vacancies (Vo) to migratetoward the top surface to compensate for the negative polarization

Fig. 2. (a) I–V curves under different times sweeping from þ6 V to 0, (b) time dependence of the open circuit voltage (Voc) with different sweeping time, (c) I–V curves underdifferent sweeping positive voltages varying from 1 V to 7 V, and (d) sweeping voltage dependence of open circuit voltage (Voc) with different sweeping positive voltages.

R.L. Gao et al. / Physica B 432 (2014) 111–115 113

charge. Hence, the migration of Vo can form a Vo-rich defectivelayer, which will remain between the Ag and BLFO layers in thecapacitor, as shown in Fig. 4.

From our results above mentioned that, migration of oxygenvacancies is the primary factor for the observed positive Voc withoutlight illumination in the Ag/BLFO/LSMO hetero-structure. As aVo-rich defective layer remains between the Ag and BLFO layersin the capacitor, acting as a cell with Ag as positive pole and LSMOas the negative pole, which can explain the positive Voc in the I–Vcurve, as shown in Fig. 1(a) and (b).

In the case of consistent positive pulses applying to the Agelectrode, more and more oxygen vacancies with positive chargesin BLFO are naturally attracted to, and thus accumulate, at theLSMO bottom electrode (polarization head) side when a highelectric field is applied, therefore the Vo concentration near the

Fig. 3. (a) I–V curves in the dark and with light illumination, (b) I–V curves with illumination of different light intensities, and (c) light intensity dependence of open circuitvoltage (Voc).

Fig. 4. Schematic energy band diagrams illustrating the distribution of oxygenvacancies (Vo) for Au/BSFO/FTO structure, the virgin BLFO samples with self-polarization at polarized down.

Fig. 5. PFM phase image of BLFO film. The filmwas poled through PFM tip scanningof the film surface with �10 V with a square area 1�1 mm2. After that, thepolarization in the center 0.5�0.5 mm2 area is scanned with þ10 V. Finally, thepiezoelectric phase image was carried out by applying an ac voltage (frequency6 kHz, amplitude 2 V peak-to-peak) to the PFM tip with a square area 1�1 mm2.

R.L. Gao et al. / Physica B 432 (2014) 111–115114

Ag top electrode decreases, results in a decreasing Voc withincreasing positive pulses, as seen in Fig. 1 (c). On the contrary,when consistent negative pulses were applied, more and moreoxygen vacancies can move forward to the Ag/BLFO interface,therefore more and more positive charges accumulate near theAg/BLFO interface, thus, Voc increases with negative pulses increas-ing, as shown in Fig. 1(d).

In the I–V sweeping process, oxygen vacancies move fromAg top electrode to LSMO bottom electrode because of discharging.Therefore, with the sweeping duration time from þ6 V back to 0 Vincreasing, i.e., as the duration of applying positive voltageincreasing, more and more Vo disappeared from Ag/BLFO layer.Hence, Voc decreases with the sweeping duration time increasing,as shown in Figs. 2(a) and (b). In consideration of the fact that Voc

was calculated from I–V curve, on the one hand, it was limited bythe maximum sweeping voltage and it impossible exceeded themaximum sweeping voltage. On the other hand, Voc shouldincrease with the maximum sweeping voltage increasing, asshown in Fig. 2(c) and (d). Oxygen vacancies with positive chargesin BLFO are naturally repelled from Ag electrode and attracted to,thus accumulate, at the LSMO bottom electrode side when a highpositive electric sweeping field is applied to the Ag electrode.Therefore, the Vo concentration near the Ag top electrodedecreased in high voltage (HV), which resulted in a decrease ofVoc. That is why Voc decreases when the maximum sweepingvoltage increases to 7 V as seen in Fig. 2(d).

In the case of light illumination, the most likely mechanismhere is that the creation of the electron–hole pairs with the holescontributing to the conduction process while the electrons aretrapped at the oxygen vacancies [27,28]. The oxygen deficiencyincreases meaning that the trapping mechanism for the electronsis formed, as oxygen vacancies can trap the photo inducedelectrons and the photo induced holes become the extra carriersthat produce the increase of photo current. This model providessimple explanations for the result that the short circuit photocurrent (Ishort) increased with more oxygen deficiency attracted tothe Ag/BLFO interface while consistent negative pulse voltageswere applied to the Ag electrode in our previous report [29].Therefore, with the light intensity increasing, more and morephoto induced electrons was trapped, and accumulated to the Ag/BLFO interface to neutralize oxygen vacancies with positivecharges, which reduced the Vo concentration and confined itsmovement, more photo induced electrons meant more oxygenvacancies with positive charges were neutralized in the Ag/BLFOinterface. As a result, the open circuit voltage decreases with lightintensity, as shown in Fig. 3(b) and (c).

4. Conclusions

In conclusion, Bi0.9La0.1FeO3 thin films have been grown onLa0.7Sr0.3MnO3/SrTiO3 substrates by using pulsed laser depositionwith Ag as the top electrode. The intrinsic open circuit voltage(Voc) (without light illumination) as large as 5.8 V was observed inthis hetero-junction structure. Influences of electric pulse treat-ment, light illumination and measurement duration time on theVoc were studied, significant Voc improvements were observedafter shortening the measuring duration time and/or consistent

negative pulses, yet Voc dropped obviously with consistent positivepulses and/or with light intensity increasing. Two stable stateswith high and low Voc.can be switched with optical switching, thismay be useful in practical application. Accumulation of oxygenvacancies with positive charges near the interface is proposed toexplain these results.

Acknowledgment

The present work has been supported by the National BasicResearch of China, the National Natural Science Foundation ofChina, the Knowledge Innovation Project of the Chinese Academyof Sciences, and the Beijing Municipal Natural Science Foundation.

References

[1] P. Ramesh, Nature 461 (2009) 1218.[2] N. Mathur, Nature 454 (2008) 591.[3] W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759.[4] R. Ramesh, N.A. Spaldin, Nat. Mater. 6 (2007) 21.[5] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland,

V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe,M. Wuttig, R. Ramesh, Science 299 (2003) 1719.

[6] T. Choi, S. Lee, Y.J. Choi, V. Kiryukhin, S.-W. Cheong, Science 324 (2009) 63.[7] W. Ji, K. Yao, Y.C. Liang, Adv. Mater. 22 (2010) 1763.[8] N.A. Spaldin, S.-W. Cheong, R. Ramesh, Phys. Today 63 (2010) 38.[9] C.M. Hung, C.S. Tu, W.D. Yen, L.S. Jou, M.D. Jiang, V.H. Schmidt, J. Appl. Phys.

111 (2012) 07D912.[10] S.Y. Yang, J. Seidel, S.J. Byrnes, P. Shafer, C.-H. Yang, M.D. Rossell, P. Yu,

Y.-H. Chu, J.F. Scott, J.W. Ager III, L.W. Martin, R. Ramesh., Nat. Nanotech. 5(2010) 143.

[11] T. Choi, S. Lee, Y.J. Choi, V. Kiryukhin, S.W. Cheong, Science 324 (2009) 63.[12] R.L. Gao, Y.S. Chen, J.R. Sun, Y.G. Zhao, J.B. Li, B.G. Shen, Appl. Phys. Lett. 101

(2012) 152901.[13] B. Chen, M. Li, Y.W. Liu, Z.G. Zuo, F. Zhuge, Q.F. Zhan, R.W. Li, Nanotechnology

195201 (2011).[14] W. Ji, K. Yao, Y.C. Liang, Phys. Rev. B 84 (2011) 094115.[15] T.L. Qu, Y.G. Zhao, D. Xie, J.P. Shi, Q.P. Chen, T.L. Ren, Appl. Phys. Lett. 98 (2011)

173507.[16] R. Moubah, O. Rousseau, D. Colson, A. Artemenko, M. Maglione, M. Viret, Adv.

Funct. Mater. 22 (2012) 4814.[17] W. Ji, K. Yao, Y.C. Liang, Adv. Mater. 22 (2010) 1763.[18] S.J. Clark, J. Robertson, Appl. Phys. Lett. 90 (2007) 132903.[19] C. Wang, K.J. Jin, Z.T. Xu, L. Wang, C. Ge, H.B. Lu, H.Z. Guo, M. He, G.Z. Yang,

Appl. Phys. Lett. 98 (2011) 192901.[20] P.W.M. Blom, R.M. Wolf, J.F.M. Cillessen, M.P.C.M. Krijn, Phys. Rev. Lett. 73

(1994) 2107.[21] H. Yang, H.M. Luo, H. Wang, I.O. Usov, N.A. Suvorova, M. Jain, D.M. Feldmann,

P.C. Dowden, R.F. DePaula, Q.X. Jia, Appl. Phys. Lett. 92 (2008) 102113.[22] S.Y. Yang, L.W. Martin, S.J. Byrnes, T.E. Conry, S.R. Basu, D. Paran, L. Reichertz,

J. Ihlefeld, C. Adamo, A. Melville, Y.-H. Chu, C.-H. Yang, J.L. Musfeldt, D.G. Schlom,J.W. Ager, R. Ramesh, Appl. Phys. Lett. 95 (2009) 062909.

[23] H. Li, Y. Xia, B. Xu, H. Guo, J. Yin, Z. Liu, Appl. Phys. Lett. 97 (2010) 012902.[24] D. Lee, S.H. Baek, T.H. Kim, J.-G. Yoon, C.M. Folkman, C.B. Eom, T.W. Noh, Phys.

Rev. B 84 (2011) 125305.[25] S.H. Baek, H.W. Jang, C.M. Folkman, Y.L. Li, B. Winchester, J.X. Zhang, Q. He,

Y.H. Chu, C.T. Nelson, M.S. Rzchowski, X.Q. Pan, R. Ramesh, L.Q. Chen, C.B. Eom,Nat. Mater. 9 (2010) 309.

[26] C.M. Folkman, S.H. Baek, C.T. Nelson, H.W. Jang, T. Tybell, X.Q. Pan, C.B. Eom,Appl. Phys. Lett. 96 (2010) 052903;

M.F. Chisholm, W. Luo, M.P. Oxley, S.T. Pantelides, H.N. Lee, Phys. Rev. Lett. 105(2010) 197602.

[27] R. Cauro, A. Gilabert, J.P. Contour, R. Lyonnet, M.-G. Medici, J.-C. Grenet,C. Leighton, Ivan K. Schuller, Phys. Rev. Lett. 63 (2001) 174423.

[28] L. Hu, Z.G. Sheng, Y.N. Huang, W.H. Song, Y.P. Sun, Appl. Phys. Lett. 101 (2012)042413.

[29] R.L. Gao, Y.S. Chen, J.R. Sun, Y.G. Zhao, J.B. Li, B.G. Shen, J. Appl. Phys. 113 (2013)183510.

R.L. Gao et al. / Physica B 432 (2014) 111–115 115